Encephalitis

Encephalitis is an inflammation of the brain tissue. The most common cause is viral infections. In rare cases it can be caused by bacteria or even fungi.

There are two main types of encephalitis: primary and secondary. Primary encephalitis occurs when a virus directly infects the brain and spinal cord. Secondary encephalitis occurs when an infection starts elsewhere in the body and then travels to your brain.

Encephalitis is a rare yet serious disease that can be life-threatening. You should call your doctor immediately if you have symptoms of encephalitis.

Many different viruses can cause encephalitis. It’s helpful to categorize the potential causes into three groups: common viruses, childhood viruses, and arboviruses.

Common viruses

The most common virus that causes encephalitis in developed countries is herpes simplex. The herpes virus typically travels through a nerve to the skin, where it causes a cold sore. In rare cases, however, the virus travels to the brain.

This form of encephalitis usually affects the temporal lobe, the part of the brain that controls memory and speech. It can also affect the frontal lobe, the part that controls emotions and behavior. Encephalitis caused by herpes is dangerous and can lead to severe brain damage and death.

Arboviruses

Arboviruses are viruses carried by insects. The type of arbovirus that’s transmitted depends on the insect. Below are different types of arboviruses:

California encephalitis (also called La Crosse encephalitis) is transmitted through mosquito bites and mainly affects children. It causes few to no symptoms.

St. Louis encephalitis occurs in the rural Midwest and southern states. It’s generally a mild virus and causes few symptoms.

West Nile virus is most often found in Africa and the Middle East. However, it can occur in the United States. It’s usually relatively mild, causing flu-like symptoms. However, it can be fatal among older adults and people with weak immune systems.

Colorado encephalitis (also called Colorado tick fever) is transmitted by the female wood tick. It’s typically a mild disease, and most people will recover quickly.

Eastern equine encephalitis is spread by mosquitoes. It affects both humans and horses. Although rare, it has a 33 percent mortality rate.

Kyasanur forest disease is transmitted through tick bites. People can also get it by drinking raw milk from goats, sheep, or cows. Hunters, campers, and farmers are most at risk for getting this disease.

You may also have a higher risk of getting encephalitis if you live in an area where mosquitos or ticks are common. Mosquitos and ticks can carry viruses that cause encephalitis. You’re more likely to get encephalitis in the summer or fall when these insects are most active.

Although the MMR (measles, mumps, rubella) vaccine has a long history of being safe and effective, in rare cases it has caused encephalitis. Approximately 1 in 3 million children who receive the vaccine develop encephalitis. However, the statistics are much more striking for children who don't receive the vaccine. Rates of encephalitis in the days before routine vaccination reached as high as 1 in 1,000. In other words, encephalitis was roughly 3,000 times more common before vaccination was available.

Your doctor will first ask you about your symptoms. They may perform the following tests if encephalitis is suspected.

Spinal tap or lumbar puncture

In this procedure, your doctor will insert a needle into your lower back to collect a sample of spinal fluid. They will test the sample for signs of infection.

Brain imaging with CT scan or MRI

CT scans and MRI detect changes in brain structure. They can rule out other possible explanations for symptoms, such as a tumor or stroke. Certain viruses have a tendency to affect certain areas of the brain. Seeing what parts of your brain are affected can help determine what type of virus you have.

Electroencephalograph (EEG)

An EEG uses electrodes (small metal discs with wires) attached to the scalp to record brain activity. An EEG does not detect the virus that causes encephalitis, but certain patterns on the EEG may alert your neurologist to an infectious source of your symptoms. Encephalitis can lead to seizures and coma in later stages. That's why the EEG is important in determining the areas of the brain that are affected and the types of brain waves that occur in each area.

Blood tests

A blood test can reveal signs of a viral infection. Blood tests are rarely performed alone. They usually help diagnose encephalitis along with other tests.

Brain biopsy

In a brain biopsy, your doctor will remove small samples of brain tissue to test for infection. This procedure is rarely performed because there’s a high risk of complications. It’s usually only done if doctors can’t determine the cause the brain swelling or if treatment isn’t working.

Anti-viral medications can help treat herpes encephalitis. However, they aren’t effective in treating other forms of encephalitis. Instead, treatment often focuses on relieving symptoms. These treatments may include:

Your outlook will depend on the severity of the inflammation. In mild cases of encephalitis, the inflammation will likely resolve in a few days. For people who have severe cases it may require weeks or months for them to get better. It can sometimes cause permanent brain damage or even death.

Encephalitis isn’t always preventable, but you can lower your risk by getting vaccinated for viruses that can cause encephalitis. Also make sure your children receive vaccinations for these viruses. In areas where mosquitos and ticks are common, use repellant, and wear long sleeves and pants. If you're traveling to an area that's known for viruses that cause encephalitis, check the Centers for Disease Control and Prevention (CDC) website for vaccination recommendations.

Medically reviewed by Tyler Walker, MD on March 31, 2017 — Written by Shannon Johnson